An optical film for compensating phase difference ascribable to birefringence of liquid crystal cells (simply referred to as “optical compensation film”, hereinafter) has widely been used in a liquid crystal display device. Among optical compensation films ever proposed with various structures, having been widely used are stretched films having transparent resins aligned by stretching. The stretched films are exemplified by those composed of cellulose resin, polycarbonate resin, and cyclic olefin resin.
Among them, an optical film having a cellulose ester (also referred to as “cellulose ester film”, hereinafter) has widely been used by virtue of its bondability to poly(vinyl alcohol) which is used for polarizers.
The cellulose ester film, however, tends to cause optical nonuniformity on the periphery of corner portions of a liquid crystal display device (frame-like nonuniformity) due to its large photoelastic coefficient. Another disadvantage relates to a large fluctuation in phase difference value in response to environmental humidity. A cyclic olefin resin film has been known as an optical compensation film with a small photoelastic coefficient, but it has only a poor bondability with poly(vinyl alcohol) which composes the polarizer.
Methods of mixing a specific polymer to the cellulose ester film have been proposed in order to improve the frame-like nonuniformity or the humidity-induced fluctuation in phase difference value. Patent Literature 1 proposes an optical film containing an acrylic polymer and a cellulose ester, which is directed to improve the frame-like nonuniformity. Patent Literature 2 discloses an optical film containing a cellulose ester and a compound with negative birefringence, which is directed to improve durability against humidity and heat. Patent Literature 3 proposes an optical film containing a cellulose ester and a specific polystyrene, which is directed to improve the moisture-induced fluctuation. The present inventors, however, found out from our investigations that when added with an increased amount of acrylic polymer or the like so as to thoroughly improve the moisture-induced fluctuation in the phase difference value and frame-like nonuniformity, unfortunately a sufficient level of phase difference value required for the optical compensation film could not achieved and moreover bondability with the polarizer was poor.
Patent Literature 4 proposes an optical film containing a cellulose ester with a small degree of substitution with acyl group. The present inventors, however, found from our investigations that the optical film became poor in the bondability with poly(vinyl alcohol) bonded as the polarizer when the total degree of substitution with acyl group was smaller than 2.0. It was also found out that the cellulose ester with a total degree of substitution with acyl group of smaller than 2.0 is distinctively eluted into a saponifying solution, resulting in coloration of the saponifying solution.
It has therefore been desired to develop an optical film which is large in the phase difference value, small in the moisture-induced fluctuation in phase difference value, small in the photoelastic coefficient, and excellent bondability with the polarizer.